As a metal wire rod to be used in electrodes for ignition plugs (central electrodes and earth electrodes) and various kinds of sensor electrodes, a metal wire rod composed of iridium or an iridium alloy (hereinafter, sometimes referred to as an “iridium wire rod”) is known. Electrodes for ignition plugs are exposed to a high-temperature oxidation environment in a combustion chamber, and therefore may be consumed due to high-temperature oxidation. Iridium is one of precious metals, has a high melting point and good oxidation resistance, and therefore can be used for a long period of time even at a high temperature. In these applications, further improvement of durability in a high-temperature atmosphere is required. It has been a common practice for improvement of the durability of an iridium wire rod to appropriately alloy additive elements such as rhodium, platinum and nickel as adjustment of the material composition. However, deterioration of other properties cannot be prevented only by improvement based on composition adjustment by alloying, and improvement of the high-temperature oxidation resistance property by composition adjustment is also necessary.
As a method for improving the high-temperature properties of a material, an approach has been made from adjustment of a material structure besides adjustment of a composition (constituent elements). For example, the present applicant has disclosed a metal wire rod composed of iridium or an iridium alloy, wherein the abundance ratio of crystals oriented in the <100> direction that is a preferential direction during processing of the wire rod is intentionally increased with attention given to the crystal orientation of metal crystals that form the wire rod (Patent Document 1).